Method for improving the oxidation resistance of chromium containing iron,cobalt and nickel base alloys



United States Patent METHOD FOR IMPROVING THE OXIDATION RESISTANCE OF CHROMIUM CONTAINING IRON, COBALT AND NICKEL BASE ALLOYS Henri Hatwell, Brussels, and Pierre Louis Moentack,

Saintes, Belgium, assignors, by mesne assignments, to Cabot Corporation, a corporation of Delaware No Drawing. Filed May 15, 1968, Ser. No. 729,372

Int. Cl. C23c 13/02 US. Cl. 117107 4 Claims ABSTRACT OF THE DISCLOSURE Method for improving the oxidation resistance of iron, cobalt and nickel base alloys which contain chromium by vapor depositing lanthanum metal on the alloy and subjecting the thus treated alloy to elevated temperatures under vacuum conditions.

This invention relates to the treatment of metal surfaces in order to improve oxidation resistance. More particularly, the present invention is directed to a method for treating the surfaces of chromium-containing cobalt, nickel and iron base alloys with lanthanum to improve the oxidation resistance of the alloys without significantly adversely affecting the mechanical properties of the alloys.

Cobalt, nickel and iron base alloys have been widely used in the past in high temperature applications such as for example, turbine blades, which require high strength at temperature and these alloys have been found to be, in general, highly effective. However, it is very important, in many high temperature situations, such as that mentioned above that the materials used be also highly resistant to oxidation.

To meet this requirement, it has been proposed to incorporate lanthanum in certain molten alloys whereby the resulting cast or wrought material is improved with respect to oxidation resistance. However, depending on the alloy composition alloyed lanthanum can lead to the formation of intermetallic phases which can adversely affect the forgeability of the material.

It is therefore an object of the present invention to provide a method for treating cobalt, nickel and iron base alloys to improve the oxidation resistance thereof without significantly adversely affecting the mechanical properties of the alloys.

It is a further object of the present invention to provide a method whereby improved oxidation resistance can be imparted to complex shapes of cobalt, nickel and iron base alloys.

Other objects will be apparent from the following description and claims.

A method in accordance with the present invention for improving the oxidation resistance of cobalt, nickel and iron base alloys comprises vapor-depositing lanthanum metal on a cobalt, nickel or iron base alloy containing from about 15 to 25% chromium and heating the thus treated alloy under vacuum conditions at an elevated temperature.

In a particular embodiment of the present invention, a sheet of cobalt base alloy, e.g., 20% Cr, 20% Ni, balance Co, is descaled and cleaned by conventional techniques. The alloy sheet is then degassed by heating under vacuum conditions and then arranged in an apparatus where it can be vapor-deposited with lanthanum. For example, the sample can be placed in a chamber which contains a source of lanthanum metal and means for vaporizing the lanthanum such as an electron bombardment type of device. With the chamber containing the cobalt base alloy evacuated, and the electron bombard- "ice ment source activated, lanthanum is vaporized and deposited on the alloy sample.

After receiving a suitable application of lanthanum the thus treated alloy sample is heated, under vacuum conditions, to an elevated temperature to cause diffusion of lanthanum into the alloy.

The resulting material exhibits substantially enhanced Oxidation resistance at elevated temperatures and retains its characteristic mechanical properties and can be readily worked into commercially useful shapes and articles.

In the practice of the present invention a wide range of cobalt, nickel and iron base alloys can be improved, as regards oxidation resistance, through the method of the present invention. Such alloys contain, in addition to Co, Ni, and Fe, and from about 1525% Co, up to 15% W and may contain minor proportions of elements customarily present in Co, Ni and Fe base alloys. The following Table I lists various specific alloys which can be treated by the present invention for the purpose of attaining improved oxidation resistance.

TABLE I 22% Cr, up to 1% W, 18% Fe, up to 1% Si, 2% Co,

9% Mo, up to 1% Mn, up to 15% C, bal. Ni 20% Cr, 15% W, up to 3% Fe, up to 1% Si, 10% Ni,

1.5% Mn, up to 0.15% C, bal. Co 21% Cr, 14% W, 2% Fe, 22% Ni, 0.1% C, 0.3% Si,

bal. Co

In providing the initial vapor deposited lanthanum coating on the cobalt base alloy, the surface being treated need not be heated above room temperature. However, substrate temperatures up to 800 C. have been satisfactorily employed in this step of the process. As to the amount of lanthanum deposited, very small amounts have been found to be effective in ultimately providing improved oxidation resistance and the deposition of from about 0.01 milligram per om. to about 1 milligram per cm. are considered satisfactory; about 0.1 to 0.5 mg./cm. is the preferred initial coating. The heating temperature range for the lanthanum vapor deposited substrate to provide diffusion of lanthanum is from about 400 C. to 800 C. with from 600 to 750 C. being preferred and a suitable vacuum for the heating step is 10 torr. with vacuums below 10* torr, being preferred. Heating times can range from 10 minutes to 10 hours with the longer times usually being used with the lower temperatures and vice versa.

The vacuum heating step desirably provides diffusion of lanthanum to a depth of about 0.1 to 1 micron, which is detectable by electron microprobic equipment.

The following examples will further illustrate the present invention.

EXAMPLE 1 An alloy 20% Cr, 20% Ni, balance cobalt was prepared by melting high purity cobalt and metal powders (better than 99.9% pure) with electrolytic chromium (at least 99.5% purity) and cast into a slab 25 mm. x 15 mm. x 100 mm. The slab was hot rolled to 0.5 mm. sheet with two intermediate 24 hour homogenization heat treatments at 1100 C. Strips mm. x 1020 mm. were cut from sheet, electrolytically polished, heat treated one hour at 1100 C. in pure argon and water quenched. After quenching the strips were again electropolished.

Four of the thus prepared samples were mounted in the vacuum chamber of a Varian VI-20 vacuum system and positioned opposite a source of lanthanum metal supported on the anode of the electron gun in the Varian device.

The vacuum chamber was evacuated to 10 torr. (10- microns) and heated at 500 C. for 24 hours for degassing. The samples were degassed by resistance heating at 800 C. for 1-8 hours.

The samples and vacuum chamber were cooled to room temperature at a vacuum of between 10 and 10 torr. (10 to 10*" microns) after which the lanthanum source was bombarded by the electron gun in the Varian device and with the vacuum in the chamber at 10" torr. (10- microns) lanthanum was deposited on the exposed surfaces of the sample. The samples were rotated 180 C. and lanthanum was deposited on the other side. The amount of lanthanum deposited was about 3 10 mg. per cm. The calculated thickness of the lanthanum deposit was about 500 A.

After the deposition of lanthanum, the samples were heated for 6 hours in the range of 600-750 C. at a vacuum of less than 10- mm. Hg (10- micron).

In the tests, the materials were suspended in an alumina crucible and exposed to air at 990 C. for 100 hours. The following Table II shows the results obtained. The La-treated samples are those of the present invention; the La-free samples were materials subjected to the same heat treatment as the materials of this invention but not exposed to lanthanum; the untreated samples were prepared from the same alloy as the others but were not subjected to the vacuum heat treatment.

TABLE II Weight gain, Sample mg./em. Oxide adherence La-Treated 0.47 Excellent. La-Free 3.81 Poor. Untreated 2. 7 Very poor (some oxide lost).

What is claimed is:

1. A method for improving the oxidation resistant alloys selected from the group consisting of cobalt, nickel and iron base alloys containing from about 15% to 25% chromium which comprises:

(1) vapor depositing lanthanum metal on a substrate of said alloy, and

(2) heating the thus treated substrate under vacuum conditions at an elevated temperature for a time sufficient to cause diffusion of lanthanum into the cobalt base alloy substrate.

2. A method in accordance with claim 1 wherein the heating is conducted at a temperature in the range of from about 400 C. to 800 C. and at a pressure of less than about 10" torr. for from 10 minutes to 10 hours.

3. A method in accordance with claim 1 wherein lanthanum is dilfused into the substrate to a depth of at least 0.5 micron.

4. A method in accordance with claim 1 wherein lan thanum is deposited on the selected alloy in an amount of between about 0.01 and 1 mg. per cm.

References Cited UNITED STATES PATENTS 3,129,069 4/1964 Hanink et a1. 29194 X 3,184,330 5/1965 Carter 29196 X OTHER REFERENCES Powell, Vapor Deposition, John Wiley & Sons, Inc., N.Y. 1966, pp. 530, 225.

ALFRED L. LEAVITT, Primary Examiner W. E. BALL, Assistant Examiner U.S. C1.X.R. 

